SDI Menu problem - Windows MSVC 6.0

The prof in our class gave this example of a windows program. When he first tried it in class, the "Sample Dialog" menu item was grayed out and inaccessible. He later got it to work, but he went through the procedure too fast to follow. When I try it, I still get the grayed out problem, as do others in the class. The prof insists it works fine. The example is this:
Use MFC AppWizard(exe) to create an SDI (Single Document) framework called MenuApp.
In Resource View, right click Dialog Resource and select Insert Dialog.
In the Properties window for the Dialog Box that pops up, change ID to "IDD_SampleDlg", caption to "Sample Dialog".
Back in Resource View, expand Menu Resource, double click IDR_MAINFRAME.
Click File on Menu Resource that is displayed, and type "Sample Dialog" in the open box.
In Sample Dialog Menu Item properties box, type "ID_SHOWDIALOG" in the ID field.
Hold CTRL & double click Sample Dialog Menu Item to bring up Class Wizard, click Add Class and New.
In Name field, enter CSampleDlg. In Base Class field choose CDialog. In Dialog ID field type ID_SHOWDIALOG.

*** I believe the problem is in the above instructions, because we can't enter ID_SHOWDIALOG in the Dialog ID field in the last step. It's not an available choice, and can't be entered by hand. ***
Later, in the OnShowDialog member function we enter:
CSampleDlg dialog;
dialog.DoModal();
and #include CSampleDlg.h in MainFrm.cpp
Any help on this? The prof isn't. Thanks.